1. Field of the Invention
The present invention relates to an electrophotography processor of a laser beam printer, a copying machine, and a facsimile system for a general sheet of paper, and more particularly to a process cartridge of an image forming device for driving a charging roller and a transfer roller by utilizing a friction portion provided on outer circumferential surfaces (periphery) of both end portions of a photosensitive drum utilizing a frictional force between the drum and rollers.
2. Description of the Related Art
FIG. 1 is a view illustrating a general construction of a prior electrophotography processor, and a printing process of the electrophotography processor will be described with reference to FIG. 1. A surface of a photosensitive drum 80 is electrically and regularly charged by a corona discharge of an electrical charger 72. The surface of photosensitive drum 80, which is electrically charged by rotating the photosensitive drum 80 with respect to electrical charger 72, receives an electric signal for forming an image through a latent image forming device 73 to thereby expose the surface of photosensitive drum 80 to a light. Thus an electrostatic latent image is formed on that potion of photosensitive drum 80 exposed to the light in an initially charging state. When the photosensitive drum portion on which the electrostatic latent image is formed passes a developing unit 74 adjacent to the surface of the photosensitive drum 80 the electrostatic latent image is developed by a toner so that the electrostatic latent image is changed into a visible image. Then, a recording sheet of paper 76 in feed cassette 77 is fed by a feeding roller 75, so that the toner image of the surface of photosensitive drum 80 is transcribed to paper 76 by a high pressure action of the transcriber 78. A separator 79 separates paper 76 from the surface of photosensitive drum 80 by an electrostatic force. Paper 76 is then transferred and passed between a pressing roller 82 and a heating roller 81 of a fixing unit, and an image is fixed on paper 76 by heat and pressure, and the resulting printed sheet of paper is transferred to a stacker 83 thus accomplishing a desiring printing operation. Afterwards, the latent toner which remains on the surface of photosensitive drum 80 after passing transcriber 78 is eliminated by a cleaner 70, and the remaining charge is eliminated by a latent image eliminating lamp 71. A problem exists however because of the use of a corona by electrical charger 72 since it requires a high voltage and generates ozone in large quantity.
Where the device of FIG. 1 utilizes a rotating photosensitive drum 80, a stationary charger 72 and a stationary transcriber 78, the device of FIG. 2 utilizes a process cartridge having rotating photosensitive drum 40, a rotating charging roller 44 and a rotating transfer roller 42 engaged to one another. In FIG. 2, photosensitive drum 40 is rotated at a certain speed, and one side of the photosensitive drum 40 is provided with a first driving gear 46 being rotated by the driving force of a main motor (not shown), and the other side of the photosensitive drum 40 is provided with a second driving gear 48 for driving a charging roller gear 50 and a transfer roller gear 52. Since second driving gear 48 is engaged with the charging roller gear 50 for rotating the charging roller 42, charging roller 42 electrically and regularly charges the surface of photosensitive drum 40 by a contact charging method thus overcoming the problem of corona discharge described above with respect to FIG. 1. Additionally, since second driving gear 48 is also engaged with transfer roller gear 52, transfer roller 44 transfers a toner image on the surface of photosensitive drum 40 to a sheet of paper.
Referring to FIG. 3, first driving gear 46 is engaged with a developing roller gear 60 for rotating a developing roller 56 for developing the toner to the electrostatic latent image formed on the surface of photosensitive drum 40. Developing roller gear 60 is further engaged with a feeding roller gear 62 for rotating a feeding roller 58 for transferring toner from a hopper (not shown) to developing roller 56.
A problem exists with respect to the device of FIG. 2, however, because second driving gear 48, charging roller gear 50 and transfer roller gear 52 are provided to drive the charging roller 42 and the transfer roller 44 thus increasing manufacturing cost, and jitter is generated during driving of the gears, and in which second driving gear 48, transfer roller gear 52 and charging roller gear 50 are provided to each roller thus increasing the number of steps needed in an assembling process thereby reducing productivity Also, there have been problems in which first driving gear 46 and second driving gear 48 tend to deteriorate a transferring regularity and a charging regularity and to deteriorate a regularity of a nip by a difference of a pressure delivered to both sides of photosensitive drum 40. Also, there has been problem in which a size of a product is large to thereby deteriorate a fine view of the product.
As an alternative to utilizing the gearing engagements described above with respect to FIGS. 2 and 3 for rotating charging roller and transfer roller, U.S. Pat. No. 5,140,371 to Yuji Ishihara et al, and entitled Contact Charging Member, Contact Charging Method Making Use Of It, And Apparatus Making Use Of It, contemplates passively rotating a charging roller and a transfer roller by keeping the charging roller and transfer roller in contact with a rotating photosensitive drum at a given pressure. Ishihara et al. proposes a contact pressure between the charging roller and the photosensitive drum be at a linear pressure of from 10 to 100 g/cm.
U.S. Pat. No. 5,235,386 to Hideyuki Yano, et al. entitled Charging Device Having Charging Member, Process Cartridge And Image Forming Apparatus, discusses problems that can occur in the above mentioned contact charging method and apparatus. Yano et al. suggests that these problems can be caused caused by toner, external additives, dust, etc., being unevenly attached to the charging roller thus causing improper charging, or charging failure, and toner fusion. Yano et al. suggests that a coefficient of friction between the photosensitive drum and the contact roller be not less than 0.4 and further suggest a pressure of 400 g between the photosensitive drum and the contact roller.
Due to a desire to increase the rotation speed of the photosensitive drum, I have found that a problem with respect to the above mentioned patents exists in that the increased rotation speed may cause a slippage between the photosensitive drum and the contact roller as well as between the photosensitive drum and the transfer roller, causing uneven charging and transfer of an image.